Striking your beer bottle straight down on someone else's beer bottle such that the concussive force causes the other's beer to erupt like a foamy volcano. A common grundy to play on a lightweight by feigning to cheers them. The word cock is an onomatopoeia for the sound of the collision.

Example of cocking a beer:
Jaime: Dude, that guy Cam is trying to pimp Tammy and he's totally a lightweight.
Robert: Yeah dude, you should go cock his beer.
Jaime: Hell ya dude.